A carboxylic acid anhydride is a useful compound not only as a starting material for organic synthesis, but also as a starting material of or a modifier for alkyd resin, unsaturated polyester resin, polyamide resin, polyimide resin, photosensitive resin, and the like, as well as a curing agent for epoxy resin. A carboxylic acid anhydride is widely used as a starting material, a resin additive, or a resin-curing agent in various fields involving adhesive agents, coating compositions, inks, toners, coating agents, molding materials, electrical insulation materials, semiconductor sealing materials, resist materials, plasticizers, lubricants, fiber treatment agents, surfactants, pharmaceuticals, agricultural chemicals, and the like.
In particular, cyclic carboxylic acid anhydrides, such as tetrahydrophthalic anhydride, hexahydrophthalic anhydride, methyl tetrahydrophthalic anhydride, methyl hexahydrophthalic anhydride, methyl nadic anhydride, and hydrogenated methyl nadic anhydride, are characterized in that they have excellent miscibility as an epoxy resin-curing agent and in that they shrink to a small degree at the time that the resin is cured, achieving excellent dimensional stability of the resulting epoxy resin molded article (cured product).
Epoxy resin-curing agents containing such a cyclic carboxylic acid anhydride have a pot life longer than that of amine-based epoxy resin-curing agents, and cured resin products obtained using the cyclic carboxylic acid anhydride have more excellent transparency, as well as more excellent high-temperature electrical properties, compared with cured resin products obtained using an amine-based epoxy resin-curing agent. Thus, cured resin products obtained using the cyclic carboxylic acid anhydride are used as insulation materials in which electronic components are casted, impregnated, and laminated, and as sealing materials for optical semiconductors (Patent Literature (PTL) 1 and Non-patent Literature (NPL) 1).
However, when an epoxy resin is cured by heating using the cyclic carboxylic acid anhydride, the cyclic carboxylic acid anhydride itself undergoes volatilization under heating, causing problems such as contamination of the heating device and significant degradation of working environment. Further, a partial loss of the acid anhydride from the resin composition as described above problematically causes an imbalance of the composition of the resin composition, making it impossible to obtain desired curing properties. In particular, in the production of a coating film or a minute molded article, an acid anhydride, which undergoes volatilization while the resin is cured by heating, has a great effect on the physical properties of the resulting cured product, making it difficult to obtain the transparency, heat resistance, and the like, of the resin. For this reason, development of an acid anhydride that does not undergo volatilization (nonvolatile acid anhydride) has been in demand. An acid anhydride undergoes volatilization while resin is cured by heating as above because an acid anhydride itself has a high vapor pressure.
In view of the above, chain (non-cyclic) polycarboxylic acid anhydrides formed by polycondensation of multivalent carboxylic acids have been developed as a replacement for cyclic carboxylic acid anhydrides. For example, chain polycarboxylic acid anhydrides obtained by an intermolecular dehydration-condensation reaction of aliphatic dicarboxylic acids, such as polyazelaic acid and polysebacic acid, have been used as a curing agent for thermosetting resins, such as epoxy resin, melamine resin, and acrylic powder coating compositions. When the chain polycarboxylic acid anhydride is used as a curing agent, the resulting cured product is likely to have excellent flexibility and excellent thermal shock resistance. Thus, the chain polycarboxylic acid anhydride is a useful compound as a curing agent for powdery coating compositions, as well as for casting resins (Patent Literature (PTL) 2).
Patent Literature (PTL) 3 discloses a compound having at least two non-cyclic acid anhydrides, the compound being represented by the following formula (I):
wherein R is a C2-50 monovalent hydrocarbon group, and R′ is a C2-50 divalent hydrocarbon group, wherein the hydrocarbon groups represented by R and R′ may contain an ether bond, a urethane bond, or an ester bond, and wherein n is an integer of 1 to 500. Patent Literature (PTL) 3 discloses that this compound is useful as a crosslinking agent (curing agent) for epoxy resin.
However, the compound specifically described in PTL 3 is a chain polyester compound (compound of Example 7) obtained from phthalic anhydride and 1,6-hexanediol, and has an aromatic ring, causing problems such that a cured product obtained using this compound is likely to be colored, and such that the glass transition temperature (Tg), which is an index of heat resistance, is significantly low.
Non-cyclic acid anhydrides have accordingly been developed; however, when these acid anhydrides are used as a resin-curing agent, the resulting resin molded article still suffers from various problems, and excellent resin molded articles are not currently obtained.